Much of today's modern Ethernet infrastructure is based on twisted pair copper cables that meet certain specifications. One common “category” of Ethernet cable is identified as CAT5e, which is rated for data rates up to 1 Gbps. Recently, however, proposals have been made to use the existing Ethernet infrastructure in the enterprise environment for data rates above 1 Gbps and up to 5 Gbps and beyond. Using cabling such as CAT5e at higher rates poses challenges such as alien crosstalk.
The Ethernet network in an enterprise environment includes many point-to-point data links. A signal on one data link may cause alien crosstalk noise into another adjacent link. CAT5e cable is generally not standardized for alien crosstalk, and the operating behavior typically varies widely from cable to cable and setup to setup. A BASE-T Ethernet channel consists of 4 pairs (sub-channels) of twisted pair copper wire, with the data being modulated on each channel. For instance, in 10GBASE-T, the data is carried over a 2-D modulated signal, with each channel carrying 7 bits of data mapped to a point in a DSQ128 constellation. Other types of constellations include SQ256, which can carry 8 bits of information, and SQ64, which can carry 6 bits of information.
Generally, in order for a receiver to detect a transmitted constellation point correctly, the signal-to-noise power ratio (SNR) should be sufficiently high. For instance, in order to meet the target error rate of 10GBASE-T, the required SNR to decode data bits carried on a DSQ128 constellation is roughly 24 dB. A denser constellation like SQ256, which carries 8 bits per symbol, needs 3 dB more SNR (27 dB) to meet the same error rate. A sparser constellation like SQ64, which carries 6 bits per symbol, needs 3 dB less SNR (21 dB) to meet the same error rate.
Noise characteristics often depend on the type of cable as well as the bandwidth of the signal. Alien crosstalk can be a dominant source of noise when the bandwidth of the signal is large and the quality of the cable is poor. CAT6a cables, which are widely used in data centers for 10GBASE-T, are higher quality cables with better balance and farther spacing between copper pairs within and without the cable. CAT5e cables are lower quality cables but widely used in enterprise environment for up to 1G bit per second of data-rate. The noise characteristics in traditional Ethernet systems (10GBASE-T and 1000BASE-T) are fairly similar across all 4 lanes of the receiver.
There are new transceivers with higher throughput (5 and 2.5 Gbps) that are targeted for Cat5e cables in an enterprise environment. The bandwidth of these new transceivers is wider than the traditional 1000BASE-T systems, making them more susceptible to alien crosstalk. The alien far-end crosstalk (AFEXT) is the dominant type of alien crosstalk in these systems. The power of alien crosstalk in a 6-around-1 configuration may vary as much as 8 dB or more from one lane to another.
In a traditional approach to BASE-T systems, the SNR of the worst channel generally needs to be high enough for the constellation used in modulation. In this approach, the channels with higher SNR are not utilized efficiently.